'The X has no access to path Y' or 'Access to the path X is denied' for File and Image Controls

using System.IO;  
...  

protected void Page_Init(object sender, EventArgs e) {  
    string dirVirtualPath = "~/TestDir";  
    string dirPhysicalPath = MapPath(dirVirtualPath);  
    if(!Directory.Exists(dirPhysicalPath)) {  
        Directory.CreateDirectory(dirPhysicalPath);  
    }  

    string fileName = "TestFile.txt";  
    string fileFullPath = Path.Combine(dirPhysicalPath, fileName);  

    File.WriteAllText(fileFullPath, "File Content Here...");  

}  

Imports System.IO  
...  

Protected Sub Page_Init(ByVal sender As Object, ByVal e As EventArgs)  
    Dim dirVirtualPath As String = "~/TestDir"  
    Dim dirPhysicalPath As String = MapPath(dirVirtualPath)  
    If (Not Directory.Exists(dirPhysicalPath)) Then  
        Directory.CreateDirectory(dirPhysicalPath)  
    End If  

    Dim fileName As String = "TestFile.txt"  
    Dim fileFullPath As String = Path.Combine(dirPhysicalPath, fileName)  

    File.WriteAllText(fileFullPath, "File Content Here...")  
End Sub